1. Field of the Invention
The present invention relates to an electrolytic capacitor of a surface mounting type.
2. Description of the Related Art
Heretofore, an electrolytic capacitor having a capacitor element, a casing made of a metal such as aluminum that holds the capacitor element, and an electrolyte solution in which the capacitor element held in the casing has been widely used.
Although electrolytic capacitors include an SMD (surface mounted device) mounted on a substrate by reflowing, and an IMD (insert-molded device) mounted on a substrate by printing, the present invention relates to an SMD-type electrolytic capacitor.
In the SMD-type electrolytic capacitor, a paste-like solder known as “cream solder” is previously printed on a substrate so as to enable surface mounting, an SMD is mounted on the surface of the substrate using an injecting molding machine known as a chip mounter, and thereafter, the substrate is heated to about 250° C. in a high-temperature furnace to melt the solder, and the SMD is adhered on the substrate.
In such an electrolytic capacitor, although the casing must be sealed so that the electrolyte solution and water vapor do not leak out of the casing, there may be a case wherein the internal pressure of the casing is elevated by the gas generated by heating during reflowing to expand the casing and to deform the SMD, and defect wherein the electrolytic capacitor cannot be properly mounted occurs.
Recently, on the other hand, the regulations on environmental contaminants have become increasingly strict, and as part of it, the movement to regulate six chemical substances, such as lead, mercury, cadmium and hexavalence chromium covering electrical and electronic equipment (RoHS Directive) is spreading across the world. The RoHS Directive prohibits the use of the six substances in the products marketed in EU member nations from July 2006, and the industry is required to respond to the RoHS regulations.
Lead, which is specified in the RoHS regulations as a hazardous substance, is contained in lead solder widely used for a long time in the reflow step for an electrolytic capacitor, and since the possibility of the elution of lead from lead-containing waste due to acid rain or the like has been pointed out, switching to unleaded solder has been desired. However, since the melting temperature of most unleaded solders presently considered is higher than before, there is no choice other than raising the reflow temperature. Therefore, the quantity of gas generated in the casing due to the rise of heating temperature during the reflow of an electrolytic capacitor tends to increase than before. Heretofore, for these problems, although the approach to seal the generated gas in the casing has been tried, there is limitation in such countermeasures.
Therefore, as an electrolytic capacitor having a structure that lets off the air out of the casing, but does not let off the electrolyte solution and water vapor in the casing, there has been disclosed an electrolytic capacitor wherein a capacitor element is held in a casing consisting of a composite resin of a phenol resin and alumina, and equipped with a gas transmitting section having fine pores each having a diameter of 0.01 to 2 μm (for example, refer to Japanese Patent Laid-Open No. 2000-286170, pp. 2-3, FIGS. 1 and 2).
However, since the electrolytic capacitor disclosed in the above reference must use a casing equipped with a gas transmitting section of a special structure, there is a problem of higher costs. In addition, there is possibility of causing environmental contamination by the gas released from the gas transmitting section.